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Method of Joining Using Gold-Rich Braze Alloys in Conjunction With Thick Gold-Plated Surfaces

IP.com Disclosure Number: IPCOM000045936D
Original Publication Date: 1983-May-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 133K

Publishing Venue

IBM

Related People

Ainslie, NG: AUTHOR [+5]

Abstract

This article describes method of brazing electronic package components together such that the braze joints thus produced exhibit melting temperatures that are substantially higher than those of the original braze alloy used in the operation. This feature is especially important when the brazing operation produces a braze joint having a melting temperature that is normally lower than the temperature at which subsequent manufacturing operations must occur. An example is the brazing of nickel-plated metal parts to nickel-plated multilayered ceramic structures at 400ŒC using Au-20 wt % Sn braze alloy (melting point 280ŒC) prior to one or more 350ŒC chip-joining operations. The problem is that the braze joint experiences partial melting at 350ŒC with its many attendant undesirable effects, included among which are:

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Method of Joining Using Gold-Rich Braze Alloys in Conjunction With Thick Gold-Plated Surfaces

This article describes method of brazing electronic package components together such that the braze joints thus produced exhibit melting temperatures that are substantially higher than those of the original braze alloy used in the operation. This feature is especially important when the brazing operation produces a braze joint having a melting temperature that is normally lower than the temperature at which subsequent manufacturing operations must occur. An example is the brazing of nickel-plated metal parts to nickel-plated multilayered ceramic structures at 400OEC using Au-20 wt % Sn braze alloy (melting point 280OEC) prior to one or more 350OEC chip-joining operations. The problem is that the braze joint experiences partial melting at 350OEC with its many attendant undesirable effects, included among which are:

1. Relative motion between the members that were

joined.

2. Diffusion of Sn from the braze joint to the

nickel-plated surfaces to form Ni-Sn

intermetallics, thus depleting the surfaces of

unreacted nickel which is essential for good

adhesion.

3. Collapse and distortion of the Au-Sn fillet

due to outdiffusion of tin and due to run-out of

the liquid phase of the braze fillet at 350^C,

with its attendant loss of strength and side

support in the case of I-O pins.

To overcome the above problems, thick gold layers (0.000l-0.002") are applied onto one or both of the surfaces to be brazed together by any appropriate means, say, electroplating, electroless plating, or silk screening, followed by any appropriate heat treatment, to insure adhesion between the gold layer and the substrate material.

The basis of the method can be understood in relation to the Au-Sn phase diagram (Fig. l). When Au-20 wt % Sn braze alloy cools to room temperature, it is comprised of two phases, the brittle AuSn compound and the ductile, gold-rich b phase. The alloy has a melting of 280OEC. Increasing the gold content of the alloy decreases the amount of AuSn present at room temperature, and it also increases the liquidus temperature thus upon reheating, say to 350OEC, the alloy melts only partially. If enough gold is added to the basic alloy, say, enough to bring it to the Au-l0Sn composition, there will be no partial melting at all at 350OEC, and there will be no brittle AuSn phase in the structure at room temperature. Therefore, by providing thick gold layers on one or both of the surfaces being brazed together, it is possible to accomplish these desirable results and to reduce or eliminate the undesirable effects enumerated previously by means of a solid-liquid reaction that would occur at the brazing temperature between the liquid braze alloy and the gold surfaces; a braze joint having a melting point much higher than the original braze alloy would be the result.

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In accordance with the method, using the example of Au-Sn brazing, the process w...